This invention relates to the production of filter means, and relates more particularly to tobacco smoke filter elements. More specifically, the instant inventive concepts are primarily concerned with producing filter means for cigarettes, although the products of this invention are generally useful as filters, particularly for tobacco smoking means, whether they be cigarettes, cigars, pipes or the like. Since filters for cigarettes are particularly commercially important, the basic embodiments of the instant invention will be discussed as they relate to the production of filtered cigarettes.
In making filters for use in connection with cigarettes and the like, a number of different properties of the resultant filter must be taken into consideration. While filtration efficiency, i.e., the ability of the filter to remove undesirable constituents from tobacco smoke, is perhaps the most important property of cigarette filters, filtration efficiency must frequently be compromised in order for the filter to possess a commercially acceptable combination of other properties, including pressure drop, taste, hardness, appearance and cost. For example, the most commonly utilized cellulose acetate filter has a relatively low filtration efficiency since increased efficiency can only be obtained either by increasing the density of the filter material or the length of the filter element, both of which produce a pressure drop across the filter which is excessive and unacceptable from a commercial standpoint.
In recent years, air dilution has become a popular technique for compensating for the relatively low filtration efficiency of cigarette filters having a sufficiently low pressure drop for commercial acceptance. The air dilution technique employs ventilating air to dilute the smoke stream from the cigarette and thereby reduce the quantity of tar and other undesirable tobacco smoke constituents drawn into the smoker's mouth for each puff or draw. The ventilating air is generally provided through a plurality of perforations in the tipping paper employed for joining the filter to the tobacco column of the cigarette, and if the filter is overwrapped with plugwrap paper, an air pervious plugwrap paper is employed.
The air dilution technique has several advantages in that it is the most economical method of reducing tar, it enables achievement of the exact amount of tar delivery desired, and it also enables removal of undesirable gas phase constituents, such as CO and NO. The major disadvantage of the air dilution technique, however, is the loss of taste, particularly when employed with low tar cigarettes containing 10 mgs or less of tar. While satisfactory improvement of the taste can, in some cases, be achieved by flavor enrichment of the cigarette tobacco, such flavor enrichment technique has been found to be relatively ineffective for producing commercially acceptable taste at tar levels below 5 mgs.
Since tobacco itself is known to be effective for filtering tobacco smoke, various filter instructions have previously been proposed in which tobacco is employed as at least a portion of the filtering material for the purpose of improving the taste properties of the filtered smoke. Such previously proposed filter constructions have employed the tobacco either in the form of fine particles or granules dispersed within a bonded matrix of the primary filtering material, as described, for example, in U.S. Pat. Nos. 2,948,282 and 3,353,543; or in the form of a separate short column of loose shredded tobacco similar to the main cigarette tobacco column and generally separated therefrom either by means of an ignition suppression disk, as described, for example, in U.S. Pat. Nos. 3,288,145 and 4,091,821, or by means of other filtering materials, as described, for example, in U.S. Pat. No. 3,858,587. For the most part, however, these constructions have proven to be too cumbersome and/or costly for large-scale production, or relatively ineffective for producing commercially acceptable taste at very low tar levels, particularly when coupled with air dilution means.